On the other hand, we're taking all this at the word of somebody who, at the very least, has made the same shitty decision over and over again for five years, and can't even justify it. They've set the dog up for abuse and also neglected it, they've failed to prioritize their niece - maybe they've also left some pertinent details from this letter such as "the dog bares its teeth at her all the time, that's how we know it's afraid of her". In which case, physical aggression is still not the answer but it's a lot more understandable without necessarily assuming this girl will then act out and hurt the baby.